In printing devices, such as impact printers, the gap between the print head and the platen is one of the key factors that determine print quality. If the gap is too large, the print head loses its proper distance for printing on the surface of a printing sheet. This results in blur and deformation of printed characters. With ink jet printers, if the gap is excessively large, an error of the ink drop arrival position caused by an error of the ejecting angle is enlarged, so that the print quality is also deteriorated. For this reason, to secure a high print quality, the platen-to-print head gap should be reduced by locating the platen as close to the print head as possible in a state that the platen is opposite the print head.
Typically, a printing sheet is transported from the sheet supply side to a printing region by a transporting mechanism, such as rollers, through a sheet transport path. The height of the inner space within the transport path is large enough to allow the printing sheet to smoothly move therethrough, thereby preventing the sheet from being jammed in the transport path. Accordingly, the printing sheet must be inserted in a narrower gap in the printing region from the large enough transport path. As a result, the leading edge of the sheet hits the platen or a member near to it, possibly causing a paper jam.
One technique to solve the problem is known. This conventional, prior art platen mechanism is illustrated in FIGS. 8 to 10. As shown, a platen 81 is disposed opposite a print head 80, which is horizontally movable. The platen 81 consists of a plate-like member having a length corresponding to a movable range of the print head 80. The platen 81 is fastened to a support member 83, which may be turned about a shaft 82. The support member 83 is mechanically coupled with a solenoid plunger 84. When driven by the plunger 84, the support member 83 and hence the platen 81 are moved to and from the print head 80.
In a print mode of the printer, the platen 81 is moved toward the print head 80 and its end face is positioned in a sheet transport path 85. Accordingly, a reduced gap G is set up between the platen 81 and the print head 80 (FIG. 9).
In a sheet insertion mode of the printer in which a sheet is inserted into a printing region (including a portion of the transport path 85 located between the platen 81 and the print head 80), the platen 81 is retracted from the transport path 85, and hence its end face is moved apart from the print head 80 and the gap G is increased. Under this condition, the printing sheet P may enter the printing location smoothly or without any interruption.
This conventional platen mechanism suffers from problems. In operation, the entirety of the combination of the platen 81 and the support member 83 is pivoted by the plunger 84. To keep the gap G constant when the printer is in the print mode, exactness is required for the dimensions of the related parts. In addition, a large space is required between the opening of the transport path 85 and the platen 81. The moving sheet is frequently caught by the large space, resulting in a paper jam. Finally, the platen 81 needs to be stable when it receives an impact from the print head 80. To this end, the output power of the plunger 84 must be large.
Another prior art platen mechanism is disclosed in JP-A8-11365. In this prior art, the gap between the print head and the platen is adjusted such that the gap is increased in the sheet insertion mode, and decreased in the print mode. An eccentrically mounted cylindrical platen is rotated to adjust the gap.
In a design where the eccentrically mounted cylindrical platen protrudes beyond the guide wall defining the transport path, the radius of the movement of the cylindrical platen is larger than that of the platen per se, so that a large gap is created between the platen and the guide wall. This large gap can easily catch the leading edge of the printing sheet, causing a paper jam.
There are cases where the leading edge of the printing sheet merely hits the guide wall portion defining the gap, not causing a paper jam. In this case, the transporting speed of the printing sheet is varied. Therefore, when the printer engages in reading information on the sheet or Magnetic Ink Character Recognition (MICR) characters, the speed variation appears as read errors.